In a broad class of laser systems, the output beam power depends on the alignment of components within the optical cavity which determine the beam path. For example, in a broad class of laser systems the output beam power depends on the alignment of the resonator mirrors. The resonator of an ion laser is often designed to operate near to instability, so that the output power of this type of laser is particularly sensitive to small changes in resonator mirror alignment. High power ion lasers are so sensitive to resonator mirror misalignment that the output power of this type of laser typically changes by one percent or more per microradian of resonator mirror rotation.
Furthermore, high power ion lasers typically convert large amounts (up to 60 kW) of electric power into heat. This waste heat is typically dissipated into a water jacket surrounding the plasma tube. Under such conditions, it is extremely difficult to prevent resonator mirror misalignment due to thermal effects.
Most conventional lasers do not include means for actively stabilizing their optical cavities. Typically, their mechanical configuration is instead carefully designed, and they are constructed using high-performance materials, so that the resonator alignment is relatively insensitive to the effect of thermal gradients on the system. However, such conventional "passively stabilized" systems are inherently limited. They do not accurately maintain mirror alignment under conditions of changing heat load, such as when the laser is first turned on, when the output power level is changed, when the plasma tube current or cooling water temperature changes, or when the temperature of the surrounding environment (i.e., laboratory or factory) changes. As a result of this limitation, in most laser systems whose gain medium produces a significant heat load, a waiting period must be allowed for the laser to warm-up or to adjust to changes. Attempts to use the laser during such waiting period will result in power drift and the need for repeated manual adjustments to the resonator mirror alignment.
It would be desirable to maintain precise resonator mirror alignment (i.e., alignment within less than one microradian) in an active manner, to assure optimum output power under all operating conditions, including when the resonator is subjected to significant mechanical noise. It would also be desirable to maintain laser beam alignment automatically, to stabilize a laser's output power soon after the laser is started up (without a lengthy warmup period) and so that the laser may then undergo long periods of unattended operation. However, until the present invention it has not been known how to accomplish both these objectives.
The inventors have recognized that a useful laser beam alignment feedback signal may be extracted from the monitored output power of a laser (or some other monitored laser operating parameter) if a low amplitude, high frequency ripple ("dither") is imposed on the laser output power. It had not been appreciated until the present invention that practical, active, beam alignment may be accomplished in the case that such ripple has amplitude sufficiently small so that the ripple does not detract significantly from operation of the laser. Accordingly, it had not been appreciated until the present invention that active beam alignment may be accomplished in a manner improving the overall performance of a laser, with imposition of an insignificantly low amplitude ripple in the laser's output power. The invention employs a servo technique with phase sensitive detection to maintain intra-cavity laser beam alignment actively. While servo techniques with phase sensitive detection have not been employed to maintain intra-cavity laser beam alignment, they have been employed in laser systems for purposes other than intra-cavity laser beam alignment.
For example, the laser system of U.S. Pat. No. 4,514,849, issued on Apr. 30, 1985 to Witte, et al. and assigned to Coherent, Inc. (the assignee of the present application), employs a servo technique with phase sensitive detection to aim an ion laser beam at a dye laser so that the ion laser beam serves to pump the dye laser. In the Witte system, a positioning mirror directs the ion laser beam at the dye laser, and a rotating wedge is placed in the ion laser beam path (outside the resonator of both the ion laser and the dye laser) between the positioning mirror and the dye laser to modulate both the horizontal and vertical components of the ion laser beam's direction of incidence at the dye laser. The dye laser output is measured and supplied to a pair of phase detectors. Square wave reference signals (having frequency and phase corresponding to the horizontal and vertical modulation signal components) are also supplied to the phase detectors. Each phase detector compares the phase angles of the signal pair it receives to generate an error signal for controlling either the horizontal or vertical alignment of the positioning mirror.
However, the Witte system neither teaches nor suggests vibrating the positioning mirror to be aligned, and the Witte system includes no means for controlling the alignment of any intra-cavity component to maintain optimal intra-cavity beam alignment.